Residential circuit breakers are described in U.S. Pat. No. 4,513,268 entitled "Automated Q-Line Circuit Breaker". The circuit breaker includes a thermal-magnetic trip unit that interrupts the circuit current upon occasion of so-called "instantaneous", "short time" and "long time" overcurrent conditions. The thermal response of the trip unit is provided by means of an extended bimetal, which is part of the circuit breaker internal current-carrying components, and is separate from the magnet that provides the trip unit magnetic response. The bimetal, in a sense, forms the primary winding of a current transformer with the magnet acting as the transformer core. A separately arranged armature unit responds to the magnetic flux generated by the magnet upon the occurrence of intense overcurrent faults to release the circuit breaker operating mechanism and thereby interrupt the circuit current.
Upon occurrence of severe overcurrent conditions, the close contact between the armature and the magnet induces a strong temporary magnetic holding flux within the armature which could prevent the armature from releasing from the magnet once the overcurrent fault has cleared.
The braid conductor that is welded to the bimetal must be carefully positioned during the assembly of the trip unit components to insure that the braid does not interfere with the movement of the armature during operation of the trip unit upon occurrence of an overcurrent condition of all magnitudes and the diameter of the braid must be sized to deter overheating and embattlement of the braid material.
The present invention improves over the performance of the state-of-the art circuit breaker described above by a reduction of the size of the trip unit components to insure cost savings as well as to control the inertia of the movable trip unit components. Further advantages are found in the shaping of the magnet as well as the movable contact arm to reduce material and labor costs while, at the same time, enhancing the overall trip unit response.